Facebook reminded me that 4 years ago I was doing QRSS3 tests with light at 4.8km over the horizon.
"Much better results this evening with 481THz light beam signals over the horizon at 4.8km. Last night I was 2.5 degrees out and signals were much weaker. Tonight QRSS3 could be comfortably copied at 20dB S/N in 0.34Hz bandwidth. At the receiving end the signal is not audible and nothing visible in the sky from the transmitter. Aiming is VERY critical just above the horizon."